1. Field of the Invention
The present disclosure relates to the field of fabricating microstructures, such as integrated circuits, and, more particularly, to immersion lithography processes and tools.
2. Description of the Related Art
The fabrication of microstructures, such as integrated circuits, requires tiny regions of precisely controlled size to be formed in a material layer of an appropriate substrate, such as a silicon substrate, a silicon-on-insulator (SOI) substrate or other suitable carrier materials. These tiny regions of precisely controlled size are generated by patterning the material layer by performing lithography, etch, implantation, deposition, oxidation processes and the like, wherein, typically, at least in a certain stage of the patterning process, a mask layer may be formed over the material layer to be treated to define these tiny regions. Generally, a mask layer may consist of or may be formed by means of a layer of photoresist that is patterned by a lithographic process, typically a photolithography process. During the photolithography process, the resist may be spin-coated onto the substrate surface and then selectively exposed to ultraviolet radiation through a corresponding lithography mask, such as a reticle, thereby imaging the reticle pattern into resist layer to form a latent image therein. After developing the photoresist, depending on the type of resist, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist. Based on this resist pattern, actual device patterns may be formed by further manufacturing processes, such as etch, implantation, anneal processes and the like. Since the dimensions of the patterns in sophisticated integrated microstructure devices are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution and overlay accuracy of the involved fabrication processes. In this respect, resolution is considered as a measure for specifying the consistent ability to print minimum size images under conditions of predefined manufacturing variations. One important factor in improving the resolution is the lithographic process, in which patterns contained in the photo mask or reticle are optically transferred to the substrate via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system, such as numerical aperture, depth of focus and wavelength of the light source used.
Due to the ongoing demand for reducing the features sizes of microstructure devices in order to increase the density of the individual elements, the resolution capability of lithography systems has been continuously increased since the resolution of a lithography system is limited by the wavelength of the exposure radiation and the numerical aperture. Accordingly, the wavelength of the exposure radiation has been continuously reduced in an attempt to further increase the resolution capability of sophisticated lithography systems. Consequently, highly complex optical systems including refractory and/or reflecting optical components have been developed for appropriate radiation wavelength, for instance of 193 nm and less. Since the numerical aperture of an imaging system depends on the index of refraction of a medium provided between the last optical components of the imaging system and the surface to be exposed, recently, immersion lithography systems have been proposed in which the exposure light is not transmitted through air or vacuum from the imaging system to the surface to be exposed, but rather an immersion lithography medium may be provided having a significantly higher index of refraction. For example, an appropriate immersion medium may be purified de-ionized water for use in conjunction with a light source of a wavelength of 193 nm, such as an argon fluorine (ArF) laser. For other exposure wavelengths, any other appropriate immersion media may be used. Consequently, immersion lithography is a very promising approach for enhancing the resolution capability on the basis of presently available optical components and exposure wavelengths. On the other hand, providing an appropriate immersion medium in the gap between the final optical components of the imaging system and the substrate surface to be exposed may be associated with additional challenges to be dealt with. For example, minor variations or non-uniformities in the index of refraction of the immersion medium may adversely affect the quality of the exposure pattern that is imaged onto the substrate surface. For example, a change of the index of refraction of the immersion medium may be caused by a non-uniform flow of the immersion medium, by changes in the density of the immersion medium, by changes in temperature of the immersion medium and the like. Moreover, a sophisticated temperature control of the immersion medium may be required due to the fact that radiation may be absorbed within the medium, thereby resulting in a corresponding temperature variation, which in turn may affect the index of refraction of the medium. Furthermore, since the immersion medium is in contact at least with the substrate surface, which may include the radiation sensitive resist material, a particle contamination of the medium may also significantly affect the overall performance of the immersion lithography tool. For example, any such contaminating particles may adhere to the surface area, for instance other substrates or any surface portions of the immersion lithography tool, thereby contributing to degrading process conditions during the processing of a plurality of substrates. For example, in an immersion tool, typically a mechanism is implemented which may provide and confine the immersion medium within the gap between the imaging system and the substrate surface, or at least a portion thereof, while also providing the required temperature control of the immersion medium. A corresponding component may frequently be referred to as immersion hood and thus comprise a surface area that may be in contact with the immersion medium, wherein the corresponding hood surface has been identified as a major source of contamination of substrates. That is, during the processing of a plurality of substrates, increasing particles may adhere to the hood surface and may also be released into the immersion medium, which may then deposit on sensitive substrate areas of the substrate and/or in sensitive areas of the imaging system. Consequently, appropriate cleaning processes may be performed on a regular basis, thereby requiring the opening of the immersion lithography tool and exposure to ambient air, which in turn may also result in a significant risk of further contamination of components of the lithography tool. Moreover, the corresponding cleaning processes may significantly contribute to the overall down time of the lithography tool, which may thus result in increased production costs since photolithography processes may represent one of the most cost-determining process modules during the fabrication of sophisticated semiconductor devices.
In view of the situation described above, the present disclosure relates to immersion lithography systems and techniques for operating the same, while avoiding, or at least reducing the effects of, one or more of the problems identified above.